Vibration sensors are utilized in a number of applications to measure acceleration and/or vibrational activity. For example, a device that senses the vibrations of a piece of machinery can be utilized to determine whether the machinery is operating properly. Such sensors are also utilized in geophysical applications and applications requiring accelerometers.
Sensors based on micromachined capacitors are known to the art. In such sensors, a capacitor having a moveable plate and a fixed plate is subjected to the vibration. The moveable plate is suspended over the fixed plate by springs. When subjected to a force, the moveable plate moves, thereby changing the distance between the two plates, and hence, the capacitance of the capacitor. The distance between the plates can be determined by measuring the capacitance of the capacitor formed by the two plates.
This type of sensor can be operated in two modes. In the first mode, the springs are essentially relaxed when the device is not being subjected to an acceleration and the distance between the plates is at some predetermined value. When the sensor is subjected to an acceleration, the moveable plate is subjected to a force that is proportional to the mass of the plate and the acceleration. The distance between the plates will either increase or decrease depending on the direction of the acceleration. The distance is constantly monitored by a measurement circuit that measures the capacitance of the capacitor formed by the plates. If the acceleration remains constant for a sufficient period of time, the plates will move to a new position in which the springs are either compressed or extended sufficiently to balance the force generated by the acceleration. The distance between the plates at this new equilibrium position can be used to provide a measure of the acceleration.
The frequency response and sensitivity of such systems depends on the spring constants of the springs that separate the moveable plate from the fixed plate. If the springs are stiff, a relatively large force will be required to provide a change in capacitance that can be measured over the noise floor of the detection circuitry, and hence, the sensitivity of the instrument is poor but large accelerations can be measured. If the springs are compliant, the plate will ring at the resonance frequency of the spring/plate system when the acceleration ceases. If the springs are compliant, small accelerations can be sensed but the maximum acceleration that can be sensed will be limited. If the springs are relatively strong, the maximum detectable acceleration will be larger but the minimum detectable acceleration will also be larger. In addition, once displaced, the plate will ring at the resonant frequency of the spring/plate system when the acceleration ceases, and hence, the device will have a “dead” time. Even in those cases in which the springs are chosen such that the system is critically damped to prevent ringing, the damping time constant places a limit on the maximum frequency response of the accelerometer
The limitations imposed by the damping time constant can be substantially reduced if the device is operated in the second mode. In this mode, a bias potential is applied between the plates to maintain the plates at a predetermined separation independent of any acceleration to which the device is subjected. The bias potential applies a force to the plates that causes the plates to move closer together by an amount that is determined by the magnitude of the potential. The force is independent of the polarity of the potential; hence, even an AC signal will cause the plates to move closer together. In this mode, a servo loop maintains the capacitance of the capacitor formed by the plates at a predetermined value by altering the bias potential to compensate for changes in the forces on the moveable plate resulting from accelerations. The magnitude of the bias signal provides a measure of the acceleration to which the device is subjected. Since the accelerometer must be able to measure both accelerations and decelerations and since the bias potential can only apply a force in one direction, a DC bias potential must be applied to the plates to compress the springs such that the springs remain compressed to some degree even when the maximum acceleration or deceleration is encountered.
It should be noted that the distance between the plates remains essentially constant in this mode, even when the sensor is subjected to an acceleration. There is a small change in the separation when an acceleration occurs that is immediately corrected by the servo circuit. Since the plates do not move any substantial distance, the damping time constant problems discussed above are substantially reduced, and hence, relatively compliant springs can be utilized to obtain high sensitivity.
The sensitivity of the sensor also depends on the noise levels in the sensing circuit. The sensing circuits typically apply a signal across the capacitor to measure the capacitance. This signal also applies a force to the moveable plate; hence, the sensing circuitry must use signals that are small compared to the bias levels that are normally placed on the plates, or the sensing circuitry must be able to separate the contribution to the change in capacitance from the sense signal from the contribution from the acceleration. To provide low noise, the cost and complexity of the sensing circuit becomes a significant factor.